The invention relates to a spin pump for conveying a liquid polymer melt.
In the spinning of synthetic yarns, a polymer melt is supplied by a spin pump to a spinneret and extruded. Such a spin pump is known, for example, from EP 0636190 and corresponding U.S. Pat. No. 5,637,331. In this spin pump, individual pumps advance the polymer melt from an inlet channel to one or more outlet channels. The individual pumps are driven by a common drive shaft which extends outside of the pump housing. For the power transmission, the drive shaft is supported in a bearing bore of the pump housing and possesses an external end for coupling to the drive. This arrangement makes it necessary to seal the gap that is formed between the drive shaft and the pump housing, while taking into account that the polymer melt has a temperature of more than 200.degree. C. To ensure a uniform temperature as well as viscosity of the melt, the pump housing may be heated. However, such high demands cannot be met by conventional seals.
In the case of the pump known from EP 0189670, it is proposed to form a seal by means of a conveying screw thread. In particular, a section of the drive shaft is provided with a spiral flute. The threaded section of the drive shaft extends through a bushing that is joined to the pump housing by flanges. In the case of this seal, the rotation of the drive shaft generates a conveying effect in the sealing gap, which returns the polymer melt to the interior of the pump. Due to the low drive speeds in the range of at most 100 rpm, the spin pumps reach only a very low peripheral speed on the drive shaft. This generates a small conveying effect, and the sealing of the gap remains inadequate.
EP 0602357 discloses a pump, wherein the conveying screw thread is provided in a bushing, through which the drive shaft extends. The bushing is inserted into a housing cover. Likewise in this arrangement, the sealing effect is dependent on the peripheral speed of the drive shaft. To this extent, this seal is unsuited for low rotational speeds. For tempering the bushing, the housing cover accommodates a channel system, through which a cooling medium flows. However, this arrangement has the disadvantage of an additional tempering device inside the pump housing as well as a high coolant consumption that is thereby necessitated.
It is therefore an object of the invention to provide a spin pump for conveying a liquid polymer melt of the initially described type with a shaft seal, which operates uniformly and is in particular independent of the drive speed.
A further object of the invention is to provide a sealing system, which does not require a cooling by a separately supplied coolant.